Liberation of Terra covers it. nominally a unit has to be knocked to 50% or below to be stricken from the records and it's assets transferred to units nominally in need of replacements but still in that effective definition. I am sure there were some units that were preserved and given life support vis a vie replacements for thier reputation, meanwhile the ones that were likely to be stricken first are ones like the Divisions financed by the Houses (Marik, Davion and Steiner did so, not sure about Kurita, Liao could not afford it)
we should also remember that numerous Auxiliaries were made from Volunteers from the Houses, Periphery Volunteers, and Amaris Opposition in the RWR, those units lived and died day to day with almost no paperwork of thier existence. Hell thier identifiers were probably "Task Force X"
the big question is Royal Divisions, arguably the symbol of the Hegemony, and whether the kept to the "All present are Hegemony Natives" requirment, or if replacements began flowing from house and Periphery oriented forces.